In induction heating devices an induction coil is supplied with an alternating voltage or an alternating current, so that in a cooking utensil coupled magnetically to the induction coil and which is to be heated, eddy currents are induced, which give rise to a heating of the utensil.
Different circuit arrangements and control methods are known for controlling the induction coil. It is common to all the circuit and method variants that they generate a high frequency control or drive voltage for the induction coil from a low frequency input supply voltage. Such circuits are referred to as frequency converters.
For converting or frequency converting, normally the input supply voltage initially is rectified with the aid of a rectifier into a direct supply voltage or intermediate circuit voltage and subsequently processed for generating the high frequency control voltage with the aid of one or more switching elements, generally insulated gate bipolar transistors (IGBTs). Normally there is a so-called intermediate circuit capacitor for buffering the intermediate circuit voltage at the output of the rectifier, i.e. between the intermediate circuit voltage and a reference potential.
A first converter variant is formed by a converter in full bridge circuit, in which two so-called half-bridges are serially looped in between the induction coil and a capacitor. The half-bridges are in each case looped in between the intermediate circuit voltage and the reference potential. The induction coil and the capacitor form a series resonant circuit.
Another converter variant is formed by a half-bridge circuit of two IGBTs, the induction coil and two capacitors, which are serially looped in between the intermediate circuit voltage and the reference potential, forming a series resonant circuit. One terminal of the induction coil is connected to a junction point of the two capacitors and its other terminal is connected to a junction point of the two IGBTs forming the half-bridge.
Both the full bridge and the half-bridge variant are comparatively expensive as a result of the large number of components required, particularly IGBTs.
An optimized variant from the costs standpoint consequently uses a single switching element or a single IGBT, the induction coil and a capacitor forming a parallel resonant circuit. The parallel resonant circuit of induction coil and capacitor are looped in serially with the IGBT between the output terminals of the rectifier and parallel to the intermediate circuit capacitor.
It is common to all the aforementioned converter variants that during a first supply half-wave the intermediate circuit capacitor is charged to a no-load voltage with an amount of a peak value of the alternating supply voltage, e.g. to 325 V in the case of a 230 V alternating supply voltage as soon as it is supplied with said supply voltage.
If no control voltage is generated for generating the induction coil power, i.e. if the switching element or elements or IGBTs are inhibited, the voltage at the intermediate circuit capacitor remains roughly constant. On starting the frequency converter, i.e. if the induction coil is driven or controlled for generating an adjustable heating power or is supplied with an alternating voltage, on switching on the IGBT or IGBTs, initially a high current flows out of the intermediate circuit capacitor into the resonant circuit and through the IGBT or IGBTs. This gives rise to an audible noise in a cooking utensil heated by the induction heating device, e.g. in the bottom of a saucepan. There is also a reduction to the service life of components supplied with the high starting current.
Thus, a problem addressed by the invention is to provide a method for operating an induction heating device with a frequency converter, which permits a reliable, component-protecting and low-noise operation of the induction heating device with limited radiated interference.